Antenna reflector and feed with absorbers to reduce back radiation to feed



G. ARMEGGIANI AND FEED Nov. 10, 1964 ANTENNA REFLECT WITH ABSORBERS TO REDUCE BACK RADIATION TO FEED 2 Sheets-Sheet 1 Filed Feb. 17, 1961 FIG. 4

INVENTOR.

Gueiono Pormeggiani BY FIG. 5

FIG.

AH y.

Nov. 10, 1964 G. PARMEGGIANI 3,156,917

ANTENNA REFLECTOR AND FEED WITH ABSORBERS TO REDUCE BACK RADIATION T0 FEED Filed Feb. 17, 1961 2 Sheets-Sheet 2 1 (vswR) db 3600 3960 4600 4:60 4200 FREQ. (MC) FIG. 7

INVENTOR. Goetuno Pur'meggioni AH y. i

United States Patent Ofifice 3,l5,9l7 Patented Nov. 10, 1964 3,156,917 ANTENNA REFLECTOR AND FEED .WHTH A73- lgggBERS T REDUGCE BAQK RADIATHGN TO Gaetano Parmeggiani, Milan, Italy, assignor to Mareiii Lenkurt S.p.A., Milan, italy Filed Feb. 17, 196i, Ser. No. 89,967 Claims priority, application Italy Feb. 22, 1960 8 Claims. (Cl. 343-482) The present invention relates to a system and device for eliminating or minimizing the reflections onto the feed line of a transmitting or receiving antenna having an illuminator and a reflector.

As far as transmitting antennas are concerned, it is known that one part of the power delivered by a feed line, cable, wave guide or similar device to the antenna is radiated, another part is dissipated and another part yet is returned towards the line by way of reflection.

In order to optimize the operation of the antenna the radiated part must be maximized and the reflected part minimized within the operating range, so that on the one hand the efliciency of the antenna is made as great as possible and, on the other hand, disturbances in the operation of the system, which are incident to the presence of these reflections, are avoided. Such disturbances, especially in the case of television or radio links with a large number of channels, become excessive from the standpoint of signal quality, even if the reflected power is of too low a level to seriously affect the efilciency. It has been found that in such cases one is not so much concerned with the loss of such a small amount of power as one is with the disturbances resulting therefrom. For this reason, even a small amount of power should, if possible, be kept from being reflected back towards the feed line.

Between the radiating surface of the antenna and the feed line there is a region which usually corresponds to the body of the antenna and which from the operational point of view may be called the power distributor. In order to reduce the power which is reflected toward the line a number of diflerent systems have been proposed. According to a first system the reduction in the reflections is obtained by means of a specific design of the power distributor. The conventional type of antenna which is based on these considerations namely the horn reflector, consists substantially of a paraboloidal reflector and of a tapered section which is attached to its lower end. This type of antenna is very diflicult to make due to its complex structure and it is also very expensive. It must be noted that the chamber formed by the tapered section favors the formation of condensation moisture which flows along its walls and enters the wave guide of the feed line which is attached to its bottom end. The disadvantages of this are well-known.

In order to avoid this, it is necessary to provide suitable fiow channels for the Water, or to provide an air-drier which avoids the possibility of condensation.

It is also known that it is possible to compensate for the reflections on the inside of the power distributor by means of regulating screws or similar means. It has been found that this last solution is suitable for a limited frequency range, but, as is well known, it is no longer suitable for a broader frequency range within which low reflection losses are required.

Every-thing that has been stated so far, and which will be stated in the following description of the present in vention, relates to the case of a transmitting antenna, but in view of the reciprocity principle, all this also holds true, with certain modifications, for receiving antennas. The object of the present invention is to obviate the above disadvantages by dissipating the reflected energy or any other reactive energy, with a system which is able to absorb in the power distributor, gradually but sufficiently, the oscillations coming from the feed line which would otherwise be returned to this line, and also by preventing the return of previously reflected oscillations to the feed line. In the case of an antenna consisting of an illuminator, or primary radiator, and a reflector, or secondary radiator, this result is achieved by providing between the illuminator mouth and the region of the reflector which would tend to return energy toward the illuminator, a structure with microwave-absorbing properties which is fluted, if possible in all directions, so that the oscillations are absorbed without reflections.

The structure should be in alignment with the projection of the margin of the illuminator mouth, taken in a direction approximately parallel to the axis of the antenna reflector, so that this structure is interposed in the way of the beam of the radiations to be absorbed.

In order to improve the absorption in. both polarizations a plurality of absorbing structures may be disposed on different levels. In any case, the structure may comprise one or more component elements. Instead of this structure, it is also possible to use an absorbing layer applied to the reflector portion on which reflections tending to be returned to the illuminator would occur.

The invention will now be described with reference to the accompanying drawings which, by way of example, refer to a preferred embodiment wherein the absorbing effect is obtained from a structure which has a plurality of components and in which the illumination of the antenna reflector is offset.

FIG. 1 shows in cross-section an assembly consisting of a reflector, an illuminator and an absorbing structure.

FIG. 2 shows a top view of an absorbing structure.

FIGS. 2A and 2B are cross-sectional views taken along the lines A-A and B-B respectively of FIG. 2.

FIG. 3 shows in top view a structure with a plurality of radially disposed elements.

FIG. 4, also in top view, shows a parallel arrangement of these elements.

FIGS. 5 and 6 show, in top view and in side view, respectively, an arrangement of superimposed stacks of a structure having parallel elements.

FIG. 7 shows the matching curve for a paraboloidal antenna with or without absorbing structure.

In FIG. 1, there is shown at 1 the antenna reflector of the type using a paraboloidal shell, at 2 the illuminator connected to the feeding equipment not shown in the figure, and at M the absorbing structure which is attached to plate 3 which, in turn, is mounted on the surface of base 4 of the reflector. The illumination is olfset, that is, the illumination axis i-i is not parallel to the axis a-a of the reflector paraboloid and the focus of the paraboloid coincides with the center of the mouth of illuminator 2.

The structure M consists of one or more elements, or solid pieces 5, of microwave-absorbing material, each of which has the form of an irregular pyramid whose base is mounted on plate 3, the latter also made of absorbing material.

FIG. 2 shows one element of pyramid form in detail.

The height of each pyramid is such that the vertex V thereof is in approximate alignment with the projection along a line substantially parallel to the axis aa of the paraboloid of the upper edge of the illuminator month In this fashion, the radiation beam whichis propagated in a direction parallel to the axis of the paraboloid is fully intercepted by the absorbing material, such as 5, and is suiiiciently absorbed. In order to attenuate the reflections of the rays coming from the illuminator, the surfaces of the pyramid or pyramids facing the illuminator have an edge, which gradually slopes down from vertex .3 V. This side of thepyramid is arranged to substantially dissipate the feeding power :prior to reflection, but only with regard to the portion which would otherwise be returned to the illuminator 2. The edge between the faces of these pyramids which are juxtaposed to the reflector may have a steeper slope because this side of the pyramid 'is designed to substantially absorb the reflected oscillations coming from the opposing side of the reflector, Which'have a much lower intensity compared with the oscillations coming directly from the illuminator, and also because possible partial reflections by these faces do not directly affect the illuminator.

In practice, the slopes of these upper edges are determined after establishing the length l of this pyramid, -i.e. -;the length of the projection of the edges on the mounting plane.

In order to obtain good absorption, the length l of the pyramid should not be smaller than to 6 wave lengths (A) of the propagated oscillation.

In the case of the structure consisting of several pyrarnids, the preferred arrangements of such elements are the radial arrangement shown in FIG. 3 or the parallel arrangement shown in FIG. 4. In both arrangements, the pyramids are mounted to the plate 3 symmetrically with respect to the principal illuminating axis ii. The maximum distance between the upper edges, facing the illuminator, of two adjacent pyramids must be slightly smaller than one half wave length (A).

If necessary two or more superimposed structures may be used, as indicated in FIGURES 5 and 6 which refer to .a parallel arrangement of three stacks of pyramids.

As may be seen from FIGURE 6 the second and third stack are mounted on another plate 3 which is parallel to plate 3 and on which the respective pyramids are disposed at 180 relatively to each other, with the bases facing each other and in an array corresponding to that of the pyramids mounted on plate 3. The Vertices of the pyramids of the second stack face the vertices of the corresponding pyramids of the first stack.

in FIGURE 7 there are shown two matching curves A and B, the first being drawn in solid lines and relating to an antenna with absorbing structure and the second being drawn in broken lines and relating to an antenna Without absorbing means. The ordinate shows the values of the standing wave ratios (S.W.R.) in decibel and the abscissa shows the frequencies in megacycles within the frequency range of 3800 to 4200 megacycles.

As will be clear from the graph, in the case of the antenna with absorbing element the curve (A) is at all points below .2 decibel. Tests have been made with a paraboloidal shell reflector having its illumination axis i-i nonparallel to the reflector axis aa. The absorbing structure consisted of four radially disposed pyramids, see FIG. 3, made from a mixture of iron powder of high permeability (/L) with a suitable binder.

The height of each pyramid was 100 millimeters and the length (l) was 550 millimeters, of which 400 millimeters corresponded to the projection of the edge of smaller slope which faces the illuminator.

The distance of the projections of the edges of two adjacent parallel pyramids onto their bases was approximately 12 millimeters. The results have been illustrated with reference to an antenna having an offset illuminator. It is clear that what has been said above is also valid for an antenna illuminated along its axis. In this case it is not necessary to dissipate the full amount of the power which would be returned to the illuminator because more complete absorption can be obtained, in this instance by compensating for the reflections on the interior power distributor with the usual arrangements.

Furthermore, reference has been made to the use of an absorbing structure, but the same results can be obtained by applying a layer of absorbing material, for instance varnish, to the reflector portion where reflections occur which are directed towards the illuminator.

What is claimed is:

1. In an antenna system of the type comprising'a reflector formed of a sectional paraboloid having a fiat horizontal ground plane and having illuminating means disposed at the focal point, offset from the focal axis of said reflector, for illuminating said reflector with radiant energy: 1 l

microwave absorbing means on said ground plane, in-

terposed in the region between said illuminating means and that part of the reflector which is substantially perpendicular to the direction of'prop'agation of the incident rays from the illuminating means, for the absorption of the unwanted radiant energy from said part of the reflector which is reflected back to said illuminating means.

2. In an antenna system of the type comprising a refiector formed of a sectional paraboloid having a list horizontal ground plane, and having illuminating means disposed at the focal point, offset from the focal axis of said reflector, for illuminating said reflector with radiant energy: 7 I

microwave absorbing means comprising first and second means on said ground plane, interposed in'theregion between said illuminating means and that part of the reflector which is substantially perpendicular to the direction of propagation of the incident rays from the illuminating means, said first means gradually absorbing the radiant energy which is propagated from said illuminating means towards said part of l the reflector, and said second means gradually absorbing the radiant energy which is reflected from said part of the reflector towards said illuminating means.

3. The combination, in an antenna system according to claim 2, wherein said microwave absorbing means comprises a structure fluted in all directions and the crest of the structure does not extend beyond the projection, taken in a direction parallel to the axis of the reflector, of the edge of the illuminator mouth.

4. The combination, in an antenna system according to claim 3, wherein said structure comprises at least one solid pyramid the base of which lies in the plane of the reflector base and the vertex of which is flush with the projection, taken in a direction parallel to the axis of the reflector, of the upper edge of the illuminator mouth, the upper edge between the faces of the pyramid opposing the illuminator having a slope more gradual than that of the edge between the faces opposing the reflector, the projection of these upper edges on the plane of the reflector base being a multiple of the wave length of the propagated radiant energy.

5. The combination in an antenna system according to claim 4, wherein said structure of microwave absorb ing material comprises a plurality of component pyramids as defined in claim 4, the pyramids being disposed in a plane in radial relation to each other and symmetrically with respect to the axis of illumination, the maximum distance between the upper edges, facing the illuminator, of two adjacent pyramids not exceeding one half of the wave length of the propagated radiant energy.

6. The combination in an antenna system according to claim 5, wherein said pyramids are arranged in a plurality of superimposed levels. t

7. The combination, in an antenna system according to claim 4, wherein said structure of microwave absorbingmaterial comprises a plurality of component pyramids as defined in claim 4, the pyramids being disposedin a plane. in parallel relation to each other and symmetrically with respect to the axis of illumination, the maximum distance between the upper edges, facing the illuminatonof two adjacent pyramids not exceeding one half of the Wave length of the propagated radiant energy.

8. The combination, in an antenna system according to References Cited in the file of this patent UNITED STATES PATENTS Tiley Mar. 8, 1949 Sabloniere Aug. 9, 1949 Robertson Feb. 27, 1951 Gardner May 20, 1952 Korman Aug. 26, 1952 Risser June 30, 1953 Crawford et a1. June 11, 1957 Saltzman et a1. June 3, 1958 Cacheris Aug. 29, 1961 Fenian Aug. 20, 1963 

1. IN AN ANTENNA SYSTEM OF THE TYPE COMPRISING A REFLECTOR FORMED OF A SECTIONAL PARABOLOID HAVING A FLAT HORIZONTAL GROUND PLANE AND HAVING ILLUMINATING MEANS DISPOSED AT THE FOCAL POINT, OFFSET FROM THE FOCAL AXIS OF SAID REFLECTOR, FOR ILLUMINATING SAID REFLECTOR WITH RADIANT ENERGY: MICROWAVE ABSORBING MEANS ON SAID GROUND PLANE, INTERPOSED IN THE REGION BETWEEN SAID ILLUMINATING MEANS AND THAT PART OF THE REFLECTOR WHICH IS SUB- 